fix(desktop): report a failed DM close instead of silently restoring the row - #7400

Refs #7310.

What the report asked for, and what the code does

#7310 reports that deleting a group DM from Desktop drops the row locally
while the channel survives on the relay. I could not reproduce a delete:
Desktop never offers one for a DM.

The one affordance on a DM row is the X, labelled Close direct message,
and it publishes kind:41012 (hide) — not kind:9008 (delete).

The reporter's CLI measurements are accurate: buzz channels delete against
a DM is rejected, deterministically, because create_dm enrols every
participant with role='member' (crates/buzz-db/src/dm.rs:181) while
kind:9008 requires owner (crates/buzz-relay/src/handlers/side_effects.rs:721).
That is #4739. Desktop simply does not take that path.

The remaining symptom — "no way back to it from the UI even though it still
exists and still receives messages" — was a real bug, and #6885 fixed it on
2026年08月27日. The report is against 0.5.19 (tagged 2026年08月25日); the fix shipped
in 0.5.22.

The silent failure that is real

What survives from the report is its subtitle. Closing a DM removes the row
optimistically and rolls it back when the relay rejects the hide, but the
handler discarded the error:

} catch {
 return;
}

hideDmMutation.error is not rendered anywhere. A reader whose relay is
unreachable, or whose membership the relay no longer recognises
(forbidden: not a member of this DM), sees the conversation flicker out and
return with no explanation, and cannot tell a transient failure from a refusal.

This reports the failure the way the sidebar's project delete already does
(SidebarProjectsSection.tsx:239), preferring the relay's own message. The
optimistic removal and its rollback are unchanged.

Verification

desktop/tests/e2e/dm-hide-failure.spec.ts drives the sidebar X with the
hide forced to fail, and asserts both halves: the error is shown, and the row
is still there afterwards.

Red/green, each on a clean dist with port 4173 killed first:

without the fix expect(locator).toBeVisible() failed
 locator('[data-sonner-toast]').filter({ hasText: ... })
 element(s) not found
with the fix 1 passed

• pnpm typecheck — clean
• pnpm exec biome check on the four touched files — no fixes applied
• Full --project=smoke suite: 1396 passed, 8 failed, 1 skipped

On the 8: none are mine. Running the same eight specs on origin/main
without this branch gives 4 failed / 127 passed, and all four reproduce here.
The rest vary between runs — re-running the two extras against unchanged code
passed them. The one that fails every time,
workflow-local-controls ... restores the caret, fails on origin/main too.

Scope

This does not make group DMs deletable; that needs #4739. It stops Desktop
from failing quietly when the relay says no.